Polyolefins such as polyethylene and polypropylene may be prepared by particle form and bulk polymerization, also referred to as slurry polymerization. In this technique, feed materials such as a monomer and catalyst are fed into the pipes of a loop reactor formed from an array of vertically or horizontally oriented pipes interconnected by elbow pipes which are typically semicircular or arcuate in shape, but which may have a horizontally-oriented section. The monomer polymerizes to form a fluid slurry formed from particles of polyolefin entrained in a liquid medium. The fluid slurry is periodically and/or continuously withdrawn from the reactor, depending on the particular process used, and routed to a separation station where the solid polyolefin particles are removed to form the final product.
In a loop polymerization operation, the fluid slurry is circulated around the loop reactor using one or more axial flow pumps. The pumps have impellers (which are also sometimes referred to as propellers in the art) that are rotatably mounted within the pipes of the reactor, and provide the motive force for circulation of the fluid slurry. The pipes are typically covered with heat exchanger jackets which cool the slurry. The flow pumps are typically (but not necessarily) located in the lower elbow pipes that conduct the slurry to the vertically oriented pipes of the reactor.
In order to maximize catalyst and reactor productivity in such loop reactors, the axial flow pumps must be capable of circulating at a high flow rate, a high concentration of solid polyolefin particles. A higher slurry velocity allows for a higher solids content, which maximizes residence time and therefore catalyst and reactor productivity.
One technique for increasing slurry circulation velocity uses a set of stationary guide vanes immediately upstream of the impeller of the pump. The guide vanes are shaped to impart a rotation to the slurry that is opposite in direction to the rotation of the impeller blades, thus increasing the rotational speed of the impeller with respect to the slurry and thereby increasing head and flow. In one design, the stationary guide vanes are mounted between the inner diameter of one of the elbow pipes of the loop reactor, and a stationary nosecone located immediately upstream of the impeller hub of the pump.
While such a guide vane design is capable of significantly improving the slurry circulation velocity through the pipes of the loop reactor, the inventors have observed that, under certain circumstances, clogging and fouling can occur on the vane edges in such a design. In particular, the inventors have observed that stringy material formed during upsets of the polymerization process can collect and bind on the leading edges of the vanes. Once such stringy material begins to collect on the edges of the vanes, the resulting deposits form sites on the vanes that promote accumulation of solid material. The resulting negative feedback cycle can cause all of the space between two or more adjacent vanes to become completely clogged, necessitating a shut-down of the loop reactor in order for the vanes to be cleared of all of the accumulated solid material.
Clearly, there is a need for a loop reactor pump assembly that is resistant to clogging when stringy material is produced during upsets in the polymerization process.